android 布局转成view,Android 自定义View及其在布局文件中的使用示例(三):结合Android 4.4.2...

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最新推荐文章于 2022-06-13 01:01:01 发布

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文章标签： android 布局转成view

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http://www.cnblogs.com/crashmaker/p/3549365.html

From crash_coder linguowu

linguowu0622@gamil.com

前言:

通过Android 自定义View及其在布局文件中的使用示例和Android 自定义View及其在布局文件中的使用示例(二),我们知道了如何使用自定义的View,以及Android绘制View的理论基础,其包含三个过程,测量View大小(通过onMeasure()方法实现),计算View位置(通过onLayout()方法实现),最后开始绘制(通过onDraw()方法实现),本篇,我们将结合Android 4.4.2_r1源码详细分析测量过程的具体实现.

在第一篇里,我们提供了一个自定义的View的源代码,现在引用一下该代码与测量相关的部分:

1 @Override

2 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

3 setMeasuredDimension(measureWidth(widthMeasureSpec),

4 measureHeight(heightMeasureSpec));

5 }

7 /**

8 * Determines the width of this view9 *10 *@parammeasureSpec11 * A measureSpec packed into an int12 *@returnThe width of the view, honoring constraints from measureSpec13 */

14 private int measureWidth(int measureSpec) {

15 int result = 0;

16 int specMode = MeasureSpec.getMode(measureSpec);

17 int specSize = MeasureSpec.getSize(measureSpec);

19 if (specMode == MeasureSpec.EXACTLY) {

20 //We were told how big to be

21 result = specSize;

22 } else {

23 //Measure the text

24 result = (int) mTextPaint.measureText(mText) + getPaddingLeft()

25 + getPaddingRight();

26 if (specMode == MeasureSpec.AT_MOST) {

27 //Respect AT_MOST value if that was what is called for by28 //measureSpec

29 result = Math.min(result, specSize);

30 }

31 }

33 return result;

34 }

36 /**

37 * Determines the height of this view38 *39 *@parammeasureSpec40 * A measureSpec packed into an int41 *@returnThe height of the view, honoring constraints from measureSpec42 */

43 private int measureHeight(int measureSpec) {

44 int result = 0;

45 int specMode = MeasureSpec.getMode(measureSpec);

46 int specSize = MeasureSpec.getSize(measureSpec);

48 mAscent = (int) mTextPaint.ascent();

49 if (specMode == MeasureSpec.EXACTLY) {

50 //We were told how big to be

51 result = specSize;

52 } else {

53 //Measure the text (beware: ascent is a negative number)

54 result = (int) (-mAscent + mTextPaint.descent()) + getPaddingTop()

55 + getPaddingBottom();

56 if (specMode == MeasureSpec.AT_MOST) {

57 //Respect AT_MOST value if that was what is called for by58 //measureSpec

59 result = Math.min(result, specSize);

60 }

61 }

62 return result;

63 }

我们可以看到:protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)是一个override的方法,它接收两个参数,通过字面意思,我们知道,这两个参数分别为宽度测量规格,高度测量规格,此时,我们会有一个疑问,这两个参数是从哪里来的?这个疑问咱们先记下来,给它编个号:Q01,暂时略过,到本文下一部分,我们就知道它的来龙去脉了.接着,我们来看onMeasure方法在本地的实现:

setMeasuredDimension(measureWidth(widthMeasureSpec),measureHeight(heightMeasureSpec));

我们跟进setMeasuredDimension(int,int)方法,看看它到底都做了些什么事情:

因为我们自定义的View是继承自View,所以我们进入View.java(源码位置:/frameworks/base/core/java/android/view/View.java)去看看有没有这个方法:

16575 /**16576 *

This method must be called by {@link#onMeasure(int, int)} to store the

16577 * measured width and measured height. Failing to do so will trigger an

16578 * exception at measurement time.

16579 *

16580 *@parammeasuredWidth The measured width of this view. May be a complex

16581 * bit mask as defined by {@link#MEASURED_SIZE_MASK} and

16582 * {@link#MEASURED_STATE_TOO_SMALL}.

16583 *@parammeasuredHeight The measured height of this view. May be a complex

16584 * bit mask as defined by {@link#MEASURED_SIZE_MASK} and

16585 * {@link#MEASURED_STATE_TOO_SMALL}.

16586*/

16587 protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {

16588 boolean optical = isLayoutModeOptical(this);

16589 if (optical != isLayoutModeOptical(mParent)) {

16590 Insets insets = getOpticalInsets();

16591 int opticalWidth = insets.left + insets.right;

16592 int opticalHeight = insets.top + insets.bottom;

16593

16594 measuredWidth += optical ? opticalWidth : -opticalWidth;

16595 measuredHeight += optical ? opticalHeight : -opticalHeight;

16596 }

16597 mMeasuredWidth = measuredWidth;

16598 mMeasuredHeight = measuredHeight;

16599

16600 mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;

16601 }

果然,我们在View.java中找到了这个方法的具体实现,通过方法说明,得知此方法必须被onMeasure()方法调用 ,来保存测量到的宽度和高度,否则的话,会在测量时引发异常.通过代码主线 ,我们知道它将传进去的两个参数赋给本地的mMeasuredWidth和mMeasuredHeight变量,以便在View类中使用;好了,此时我们该抽离出来,回到我们出发的地方:

setMeasuredDimension(measureWidth(widthMeasureSpec),measureHeight(heightMeasureSpec));

有了上面的分析过程,我们知道这个方法中的measureWidth(widthMeasureSpec)是作为测量到的宽度,measureHeight(heightMeasureSpec)是作为测量到的高度,而这两个是需要我们在自定义的View中去实现的,由于测量宽度与高度的过程类似,我们在此文中仅分析measureWidth()的过程,很自然地,我们看看本地的measureWidth()是如何实现的:

1 /**

2 * Determines the width of this view3 *4 *@parammeasureSpec5 * A measureSpec packed into an int6 *@returnThe width of the view, honoring constraints from measureSpec7 */

8 private int measureWidth(int measureSpec) {

9 int result = 0;

10 int specMode = MeasureSpec.getMode(measureSpec);

11 int specSize = MeasureSpec.getSize(measureSpec);

13 if (specMode == MeasureSpec.EXACTLY) {

14 //We were told how big to be

15 result = specSize;

16 } else {

17 //Measure the text

18 result = (int) mTextPaint.measureText(mText) + getPaddingLeft()

19 + getPaddingRight();

20 if (specMode == MeasureSpec.AT_MOST) {

21 //Respect AT_MOST value if that was what is called for by22 //measureSpec

23 result = Math.min(result, specSize);

24 }

25 }

27 return result;

28 }

该方法用来确定我们自定义的这个View的宽度,它接收onMeasure()的widthMeasureSpec参数,接着

int specMode = MeasureSpec.getMode(measureSpec);

int specSize = MeasureSpec.getSize(measureSpec);

MeasureSpec.getMode(measureSpec),getMode()?我们在上一篇文章中的最后,有如下描述:

MeasureSpec:

该对象封装了父容器传递给子元素的布局要求,它有三种模式：

UNSPECIFIED:父容器对子元素没有要求，子元素可以得到任意值；

EXACTLY:父窗口决定子元素的大小，子元素将被限定在给定的边界里而忽略它本身大小;

AT MOST:子元素至多达到父窗口指定的大小,子元素不能超过这个边界;

所以我们会想,getMode()方法,应该就是获取上述这三种模式之一吧?我们跟进源码,看看getMode()都做了哪些事情:

18341 /**18342 * Extracts the mode from the supplied measure specification.

18343 *

18344 *@parammeasureSpec the measure specification to extract the mode from

18345 *@return{@linkandroid.view.View.MeasureSpec#UNSPECIFIED},

18346 * {@linkandroid.view.View.MeasureSpec#AT_MOST} or

18347 * {@linkandroid.view.View.MeasureSpec#EXACTLY}

18348*/

18349 public static int getMode(int measureSpec) {

18350 return (measureSpec & MODE_MASK);

18351 }

由此方法的文字描述部分,我们得知,该方法从接收的参数measureSpec中,获取到对应的三种模式之一,即返回measureSpec & MODE_MASK,这里的MODE_MASK又是个什么东西呢?在View.java中,我们找到在View这个类中,有个内部类MeasureSpec类

18289 public static class MeasureSpec {

18290 private static final int MODE_SHIFT = 30;

18291 private static final int MODE_MASK = 0x3 << MODE_SHIFT;

..............................................................

18297 public static final int UNSPECIFIED = 0 << MODE_SHIFT;

18298

18299 /**

18300 * Measure specification mode: The parent has determined an exact size

18301 * for the child. The child is going to be given those bounds regardless

18302 * of how big it wants to be.

18303 */

18304 public static final int EXACTLY = 1 << MODE_SHIFT;

18305

18306 /**

18307 * Measure specification mode: The child can be as large as it wants up

18308 * to the specified size.

18309 */

18310 public static final int AT_MOST = 2 << MODE_SHIFT;

................................

}

所以,MODE_MASK的值为0x3左移了MODE_SHIFT(30)位,那么,用32位的二进制来表示的话,MODE_MASK为:1100 0000 0000 0000 0000 0000 0000 0000;如果非要探究此时的measureSpec & MODE_MASK后的值是多少,那么我们不妨用Debug模式调试一下我们的代码来获取getMode方法中传进来的参数measureSpec是什么值, 首先,从上面的源码中,可以知道三种MeasureSpec三种模式的值:

UNSPECIFIED = 0 << MODE_SHIFT;即:UNSPECIFIED为:0000 0000 0000 0000 0000 0000 0000 0000

其实我们也可以参阅官方文档对此值的定义:

public static final int UNSPECIFIED

Measure specification mode: The parent has not imposed any constraint on the child. It can be whatever size it wants.

Constant Value: 0 (0x00000000)

注:只不过官方文档此处用十六进制表示而已,以下两个模式也都用十六进制表示而已.

EXACTLY = 1 << MODE_SHIFT;即 EXACTLY为:0100 0000 0000 0000 0000 0000 0000 0000

public static final int EXACTLY

Measure specification mode: The parent has determined an exact size for the child. The child is going to be given those bounds regardless of how big it wants to be.

Constant Value: 1073741824 (0x40000000)

AT_MOST = 2 << MODE_SHIFT;即 AT_MOST为:1000 0000 0000 0000 0000 0000 0000 0000

public static final int AT_MOST

Measure specification mode: The child can be as large as it wants up to the specified size.

Constant Value: -2147483648 (0x80000000)

MODE_MASK为:1100 0000 0000 0000 0000 0000 0000 0000

好,我们来看一下debug前,自定义的View在布局文件中的layout_width的配置及我所调试的设备的屏幕像素为480*800,也就是我的显示屏宽为480像素;

android:id="@+id/customView"

android:layout_width="wrap_content"android:layout_height="wrap_content"app:colorValue="@color/textRed"app:textSize="20sp"app:textString="This the Custom View1!!!" />

调试结果出来,此时传入的measureSpec的值是-2147483648,到了这里,我们又会产生一个疑问 ,为什么是它?为什么是这个值?我们先把这个疑问做个标记:Q02;到了文章最后,这个疑问就能解开了,这里先把思路跳出来,继续分析我们的measureWidth()这个本地方法的代码;

1 /**2 * Determines the width of this view

3 *

4 *@parammeasureSpec

5 * A measureSpec packed into an int

6 *@returnThe width of the view, honoring constraints from measureSpec

7*/

8 private int measureWidth(int measureSpec) {

9 int result = 0;

10 int specMode = MeasureSpec.getMode(measureSpec);

11 int specSize = MeasureSpec.getSize(measureSpec);

13 if (specMode == MeasureSpec.EXACTLY) {

14 //We were told how big to be

15 result = specSize;

16 } else {

17 //Measure the text

18 result = (int) mTextPaint.measureText(mText) + getPaddingLeft()

19 + getPaddingRight();

20 if (specMode == MeasureSpec.AT_MOST) {

21 //Respect AT_MOST value if that was what is called for by

22 //measureSpec

23 result = Math.min(result, specSize);

24 }

25 }

27 return result;

28 }

上面我们已经分析到第10行,由于第11行是获取传入的measureSpec的大小,过程与获取传入的measureSpec的模式类似,这里暂时先略过,接下来看第13行代码,这里要对获取到的模式进行判断,由上一篇文章,我们知道,如果自定义的View在布局文件中指定固定大小,那么,它的模式就是属于MeasureSepc.EXACTLY,此时,measureWidth()这个本地方法就返回11行所得的大小,否则进入另外一个分支,因为本系列中我们实现的实现上是一个类似于TextView的自定义控件,那么,这个View的大小就应该由它所绘制的文字长度来决定,此时,我们先计算出文字的宽度,然后再对其模式进行判断,如果模式是属于measureSpec.AT_MOST,我们通过数学运算,比较文字长度与通过传入的measureSpec所包含的大小,它们之中更小的那个做为我们控件的宽度.

文章开头的相关代码中,本地方法:getMeasureHeight()的过程与本地方法getMeasureWidth()类似,在此不再分析.

在此总结一下,文章开头引用的代码是我们在编写自定义View时,在重写onMeasure()这个方法时的一般步骤,那么,本文中的分析过程中还留有两个疑问:

Q01:

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)是一个override的方法,它接收两个参数,通过字面意思,我们知道,这两个参数分别为宽度测量规格,高度测量规格,此时,我们会有一个疑问,这两个参数是从哪里来的?

Q02:

调试结果出来,此时传入的measureSpec的值是-2147483648,到了这里,我们又会产生一个疑问 ,为什么是它?为什么是这个值?

要探究这两个疑问,我们在本系列第二篇文章中,曾经提过Android绘制View的理论基础,从那篇文章中,我们明白,Android要绘制View的时候,必须要先遍历View的树形结构,并且先从最顶端的结点开始遍历,通过查找官方文档,我们进入

ViewRootImpl.java(文件位于:/frameworks/base/core/java/android/view/ViewRootImpl.java),一起找出上面的那两个疑问.........

/*********************************友情提醒:开始下面的探究前,最好先休息一下*********************************/

我们先大致浏览一下ViewRootImpl.java,这个文件代码有6707行有没有,不用怕,我们先找到一个叫performtraversals()的方法,看这字面意思,它是要开始遍历的节奏啊,果断跟进去看一下,顺便找找几个有用的干货:

private void performTraversals()

{

.......................................

1122 WindowManager.LayoutParams lp = mWindowAttributes;//详见分析PERFORMTRAVERSALS()点1

.........................................................

1155 Rect frame = mWinFrame;//详见分析PERFORMTRAVERSALS()点2

.......................................................

1563 if (mWidth != frame.width() || mHeight != frame.height()) {

1564 mWidth = frame.width();

1565 mHeight = frame.height();

1566 }

1567

.......................................................................

PERFORMTRAVERSALS()点3:

1634 if (!mStopped) {

1635 boolean focusChangedDueToTouchMode = ensureTouchModeLocally(

1636 (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);

1637 if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()

1638 || mHeight != host.getMeasuredHeight() || contentInsetsChanged) {

1639 int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);//详见getRootMeasureSpec()方法的分析

1640 int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);

1641

1642 if (DEBUG_LAYOUT) Log.v(TAG, "Ooops, something changed! mWidth="

1643 + mWidth + " measuredWidth=" + host.getMeasuredWidth()

1644 + " mHeight=" + mHeight

1645 + " measuredHeight=" + host.getMeasuredHeight()

1646 + " coveredInsetsChanged=" + contentInsetsChanged);

1647

1648 //Ask host how big it wants to be

1649 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

}

/************************************分析PERFORMTRAVERSALS()点1 开始**********************************/

这里的lp用得还挺多,也许对我们有用,

因为

WindowManager.LayoutParams lp = mWindowAttributes;

所以我们分析一下这个mWindowAttributes是何方神圣:

分析PERFORMTRAVERSALS()点1:mWindowAttributes相关代码：

final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();

我们进入WindowManager类的内部类LayoutParams的构造方法

1 public LayoutParams() {

2 super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);

3 type = TYPE_APPLICATION;

4 format = PixelFormat.OPAQUE;

5 }

其中有这么一句:注意两个参数都为LayoutParams.MATCH_PARENT

super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);

因为WindowManager类的内部类LayoutParams继承自ViewGroup.LayoutParams,所以进入ViewGroup的内部类LayoutParams看一下

/frameworks/base/core/java/android/view/ViewGroup.java：

5829 public static class LayoutParams {

5830 /**5831 * Special value for the height or width requested by a View.

5832 * FILL_PARENT means that the view wants to be as big as its parent,

5833 * minus the parent's padding, if any. This value is deprecated

5834 * starting in API Level 8 and replaced by {@link#MATCH_PARENT}.

5835*/

5836 @SuppressWarnings({"UnusedDeclaration"})

5837 @Deprecated

5838 public static final int FILL_PARENT = -1;

..........................................

5918 public LayoutParams(int width, int height) {

5919 this.width = width;

5920 this.height = height;

5921 }

分析总结:这里的width与height,都被赋为LayoutParams.MATCH_PARENT,所以这里的lp的宽与高,都为LayoutParams.MATCH_PARENT

/************************************分析PERFORMTRAVERSALS()点1 结束**********************************/

######################################################################################################################

/************************************分析PERFORMTRAVERSALS()点2 开始**********************************/

1563 if (mWidth != frame.width() || mHeight != frame.height()) {

1564 mWidth = frame.width();

1565 mHeight = frame.height();

1566 })

此时的mWidth为ViewRootImpl的变量,在这里使它的值为frame.width()的值;

frame又是从哪里来的呢?在performTraversals()方法中,1155行,原来它只是个局部变量,

1155 Rect frame = mWinFrame;

到了这里,关键就是找出mWinFrame了,继续找mWinFrame:

在ViewRootImpl的变量声明中:

256 final Rect mWinFrame; //frame given by window manager.

在ViewRootImpl这个类的构造方法中:

360 mWinFrame = new Rect();

frame given by window manager?那大概就是说mWinFrame是由窗口管理类来赋值的了,那么这么里mWinFrame应该就是屏幕的窗口大小了.我们这里先这么假设,后续文章再进行验证.

/**********************************************分析PERFORMTRAVERSALS()点2 结束**************************************/

/**********************************************分析PERFORMTRAVERSALS()点3开始**************************************/

PERFORMTRAVERSALS()点3:performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

1,两个参数:childWidthMeasureSpec,childHeightMeasureSpec分析

a)childWidthMeasureSpec:

int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);

先分析getRootMeasureSpec的两个参数:

1)mWidth:

见分析点2(

1563 if (mWidth != frame.width() || mHeight != frame.height()) {

1564 mWidth = frame.width();

1565 mHeight = frame.height();

1566 })

所以猜想mWidth就是窗口的初始宽度(本文暂未验证)

2)lp.width:这里的lp就是分析点1中的 WindowManager.LayoutParams lp = mWindowAttributes;即:lp.width为LayoutParams.MATCH_PARENT;

由以上1)和2),我们先搞定了getRootMeasureSpec(mWidth,lp.width)这个方法的两个参数的意义,接下来,我们进入getRootMeasureSpec(mWidth,lp.width)这个方法

b)childHeightMeasureSpec:

int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);

先分析getRootMeasureSpec的两个参数:

1)mHeight:类似上述的猜想,这里的mHeight就是窗口的初始高度

2)lp.height:这里的lp就是分析点1中的 WindowManager.LayoutParams lp = mWindowAttributes;即:lp.height为LayoutParams.MATCH_PARENT;

因为上述a)与b)的调用过程类似,只不过a)是获取宽度的规格,b)是获取高度的规格,所以以下分析只以获取宽度规格的过程来分析

*******************************************************************进入getRootMeasureSpec()方法的分析**********************************************

1924 private static int getRootMeasureSpec(int windowSize, int rootDimension) {

1925 int measureSpec;

1926 switch (rootDimension) {

1927

1928 case ViewGroup.LayoutParams.MATCH_PARENT:

1929 //Window can't resize. Force root view to be windowSize.

1930 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);

1931 break;

1932 case ViewGroup.LayoutParams.WRAP_CONTENT:

1933 //Window can resize. Set max size for root view.

1934 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);

1935 break;

1936 default:

1937 //Window wants to be an exact size. Force root view to be that size.

1938 measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);

1939 break;

1940 }

1941 return measureSpec;

1942 }

此方法接收的第二个参数rootDimension,就是lp.width,通过上面的分析,lp.width=LayoutParams.MATCH_PARENT,所以,进入第一个switch分支

此方法的返回值measureSpec=MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);

所以,分析此方法,我们也知道,当我们的自定义View的layout_width/layout_height设置成MATCH_PARENT时,MODE 为MeasureSpec.EXACTLY;当设置成WRAP_CONTENT时,MODE为MeasureSpec.AT_MOST;

接下来我们分析1938行:

1938 measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);

***********************************************************************************************************************************************************

*******************************************************************进入MeasureSpec.makeMeasureSpec()方法的分析**********************************************

17245 /**17246 * Creates a measure specification based on the supplied size and mode.

17247 *

17248 * The mode must always be one of the following:

17249 *

17250 *

{@linkandroid.view.View.MeasureSpec#UNSPECIFIED}

17251 *

{@linkandroid.view.View.MeasureSpec#EXACTLY}

17252 *

{@linkandroid.view.View.MeasureSpec#AT_MOST}

17253 *

17254 *

17255 *@paramsize the size of the measure specification

17256 *@parammode the mode of the measure specification

17257 *@returnthe measure specification based on size and mode

17258*/

17259 public static int makeMeasureSpec(int size, int mode) {

17260 return size + mode;

17261 }

此方法在/frameworks/base/core/java/android/view/View.java中的内部类MeasureSpec中的方法,该方法返回两个参数size+mode之和,参数size对应我们传进来的windowSize,即:窗口的初始宽度(当传进来的是mHeight时,为窗口的初始高度);

参数mode对应我们传进来的MeasureSpec.EXACTLY

**************************************************************************************************************************************************************************

有了上面这些分析之后,我们可以进入performMeasure(childWidthMeasureSpec, childHeightMeasureSpec)的分析了:

1913 private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {

1914 Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");

1915 try {

1916 mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);

1917 } finally {

1918 Trace.traceEnd(Trace.TRACE_TAG_VIEW);

1919 }

1920 }

*************************************mView.measure(childWidthMeasureSpec, childHeightMeasureSpec)的分析************************************************************

16450 /**16451 *

16452 * This is called to find out how big a view should be. The parent

16453 * supplies constraint information in the width and height parameters.

16454 *

16455 *

16456 *

16457 * The actual measurement work of a view is performed in

16458 * {@link#onMeasure(int, int)}, called by this method. Therefore, only

16459 * {@link#onMeasure(int, int)} can and must be overridden by subclasses.

16460 *

16461 *

16462 *

16463 *@paramwidthMeasureSpec Horizontal space requirements as imposed by the

16464 * parent

16465 *@paramheightMeasureSpec Vertical space requirements as imposed by the

16466 * parent

16467 *

16468 *@see#onMeasure(int, int)

16469*/

16470 public final void measure(int widthMeasureSpec, int heightMeasureSpec) {

.....................................................

16496 //measure ourselves, this should set the measured dimension flag back

16497 onMeasure(widthMeasureSpec, heightMeasureSpec);

16498 mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;

16522 }

这里的measure()方法是个final方法,结合该方法的说明,

The actual measurement work of a view is performed in onMeasure()

并且measure的两个参数同时传入onMeasure()中,

所以,才有了文章开头时引用的代码,在自定义的View中,重写onMeasure()方法,那么,本文上部分遗留下来的两个问题,至此就有了答案:

Q01:

通过:1639 int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);//详见getRootMeasureSpec()方法的分析,onMeasure的第一个参数widthMeasureSpec就是这里的childWidthMeasureSpec,heightMeasureSpec对应 childHeightMeasureSpec;

Q02:

调试结果出来,此时传入的measureSpec的值是-2147483648,到了这里,我们又会产生一个疑问 ,为什么是它?为什么是这个值?

那么这里的measureSpec就是MeasureSpec.makeMeasureSpec()方法的分析中,返回的size+mode;size是手机显示屏的像素宽或者高,文章上半部分中,我调试的手机像素宽是480,而且在自定义的View的布局文件中,layout_width设置成wrap_content,通过上面的分析,当设置成wrap_content时,模式为AT_MOST模式,通过文档描述,它的十进制值是-2147483648,那么size+mode就是480+(-2147483648)=-2147483168,也就是我们调试出来时,所得到的值-2147483648

)

另外,我们或许还会有一个疑问 :为什么MODE_MASK是1100 0000 0000 0000 0000 0000 0000 0000?EXACTLY为:0100 0000 0000 0000 0000 0000 0000 0000?

AT_MOST为:1000 0000 0000 0000 0000 0000 0000 0000?

其实对于这个问题,我们想,既然android规定了MODE必须是EXACTLY,AT_MOST,UNSPECIFIED这三种模式之一,那么,就可以用32位二进制的最高两位来表示,它有00,01,10,11这四种情况,那么它的MODE_MASK取值为

1100 0000 0000 0000 0000 0000 0000 0000就能很方便地取到它的模式了,由getMode()的实现:

return (measureSpec & MODE_MASK);

我们就可以取到它的最高两位,由此来确定它是哪种模式;同理对于getSize():

public static int getSize(int measureSpec) {

return (measureSpec & ~MODE_MASK);

}

对于屏幕宽度,再大的屏幕也用不了32位二进制来表示其尺寸,所以才有measureSpec & ~MODE_MASK,这样就能取到它的值了.

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